The present invention relates generally to a flow drilling process and tool, and more particularly to a flow drilling process and tool for reliably forming a strong, uniform rimmed hole or boss in a metal workpiece.
Various types of piercing and drilling processes and tools have been devised in the past for making rimmed or bossed holes in metal workpieces. For example, in U.S. Pat. Nos. 1,813,152 and 1,906,953 issued in the name of W. L. Enghauser in 1931 and 1933, respectively, a method and apparatus are disclosed for forming tap holes in relatively thin-walled gas pipe. The pipe is predrilled to center a rapidly rotating spinning tool which heats the metal to a forming temperature and spreads the metal along the tool to form bosses in both directions. A process and tool similar to that shown by Enghauser is described in U.S. Pat. No. 2,991,551 issued to H. D. Fogle et al in 1961. However, to form more uniform peripheral rims in a single pass, Fogle discloses a tubular slug forming tool having a sharpened cylindrical cutting end. In U.S. Pat. No. 3,429,171 issued to J. J. Feher in 1969, a spinning tool is described which includes an end portion tapering to a sharp point for piercing relatively soft metal without heating.
The above-mentioned processes and tools are primarily for forming holes in relatively soft metals and are not well adapted for use with harder metals. Processes requiring predrilling usually require at least one extra step, and centering the holes with respect to the workpiece is a common problem if precise hole location is necessary. Similarly, a tubular slug-forming tool such as shown in Enghauser, although capable of reducing rim and hole length irregularities, is difficult to center, and the cutting end requires frequent sharpening when used with anything but the softest materials. The slug can become lodged in the tubular cutting portion.
More recently a flow drilling process and tool were devised for forming holes and bushings in harder metals such as iron and mild steel. The tool, described in U.S. Pat. No. 3,939,683 issued to J. A. Van Geffen in 1976, has a specially shaped conical nose with several shallow flats produced axially along its periphery. The tool and process is further described in an article entitled "Make Holes and Bushings in One Operation" appearing at pages 117-121 of the October 1979 issue of the Machine and Tool Blue Book incorporated herein by reference for background information. The rim or boss portion of the bushing formed by the Van Geffen process on the downward side of the workpiece tapers to a wavy or relatively ragged surface with numerous locations where cracks or stress risers form. As a result, bushing length is irregular, and shear stress can easily crack the bushing. Metal flow above and below the hole formed by the initial penetration of the tool is relatively fixed at about 40 percent upward and 60 percent downward, and controlling bushing length has required a separate step such as trimming, predrilling or countersinking. The tool must be specially fabricated to achieve the desired cross-sectional configuration with the shallow flats along the periphery.
It is therefore an object of the present invention to provide an improved flow drilling process, and tool therefor.
It is another object to provide a flow drilling process, and tool therefor, which overcome the problems set forth above.
It is a further object to provide a flow drilling process, and tool therefor, for providing a bossed or rimmed hole of uniform length and increased strength. It is another object to provide such a process and tool which obviate predrilling or trimming steps, and wherein the tool has increased lifetime and does not require sharpening.
It is still another object to provide flow drilling process and tool wherein the length and axial location of the hole, as well as wall thickness, can be predictably controlled. It is a further object to provide such a process and tool wherein the tool can be accurately centered on the workpiece to precisely locate the hole.
It is also an object of the invention to provide a flow drilling tool which is capable of forming a bossed or rimmed hole devoid of ragged and excessively tapered surfaces. It is another object to provide such a tool, and a process utilizing the tool, for forming the hole in a single operation without need to predrill or to cut a slug with a sharpened cutting end.
It is yet another object to provide a tool for efficiently and cleanly punching a circular slug from a relatively hard metal workpiece. It is a further object to provide such a tool with a forming portion for increasing the axial length and diameter of the punched hole, wherein the forming portion is substantially circular in cross section to simplify fabrication of the tool.
A flow drilling tool includes a cylindrically-shaped prepunch portion having a relatively blunt cone-shaped free end terminating in an apex located on the axis of the tool. Preferably the apex angle is between 60 and 85 degrees. A circular-forming portion located axially rearwardly of the prepunch portion tapers radially outwardly therefrom to an enlarged end having a radius approximately equal to the desired radius of the finished hole.
The flow drilling tool is rotated at relatively high speed as it is forced with constant pressure against a metal workpiece. Initially the apex locates the tool on the workpiece, and, as the prepunch portion is rotated, the metal softens adjacent the free end of the prepunch portion. The constant axial pressure applied to the tool and acting against the softened metal forces a circular slug from the workpiece. The resulting punched hole is enlarged radially and axially as the forming portion heats and spins the adjacent metal to form bosses or rims on each side of the workpiece.
Boss or rim integrity and appearance and hole length uniformity are improved by removing, in the form of the circular slug, the metal which is first softened by the tool and which would form a ragged edge if not removed. The blunt prepunch requires no sharpening, and the diameter of the prepunch can be varied to change the axial length of the finished hole. The final location of the hole can be controlled more closely than with a process wherein pilot holes are first drilled, and the hole can be completely formed in a single operation with one stroke of the rotating tool.
The flow drilling process also includes the step of selectively preheating an area of the workpiece, as well as controlling tool speed and feed pressure, to control metal flow above and below the workpiece. This allows all or nearly all of the metal shaped by the forming portion to be used for the wall thickness and eliminates need for trimming. Also, the location of the final attachment point of the slug to the workpiece is controlled by preheating the workpiece off-center from the axis of the tool so that axially aligned bushings can be simultaneously formed in opposed tubing walls by a pair of tools without slug interference.
While the bushing is still hot from the forming process, a quenching step can be performed to provide a hard bearing surface without a subsequent reheating step. A shield is interposed between the workpiece and the flow drilling tool, and the finished hole is sprayed with a quench solution.
These and other objects, features and advantages of the present invention will become apparent from the description of the preferred embodiment and from the drawings.